How should I read and interpret urinalysis results?

How to Read and Interpret Urinalysis Results

Urinalysis interpretation requires systematic evaluation of physical, chemical, and microscopic parameters, always correlating findings with clinical symptoms to distinguish true infection from colonization or contamination. 1

Core Components of Urinalysis

Physical Examination

• Color and clarity: Cloudy urine often results from precipitated phosphate crystals in alkaline urine rather than infection, though pyuria can also cause cloudiness. 2
• Odor: A strong odor typically indicates a concentrated specimen rather than urinary tract infection. 2
• Specific gravity: Provides reliable assessment of hydration status (normal range 1.005-1.030). 2

Chemical Dipstick Testing

Leukocyte Esterase

• Sensitivity 83% (range 67-94%), specificity 78% (range 64-92%) for detecting UTI. 1
• Positive result indicates pyuria (≥10 WBCs/high-power field) but requires clinical correlation. 1
• Negative leukocyte esterase has excellent negative predictive value (82-91%) for ruling out UTI. 1
• False-positives occur with contaminated specimens, oxidizing agents, and certain medications. 1
• False-negatives may occur with high urinary glucose, high specific gravity, or certain antibiotics. 1

Nitrite Test

• Sensitivity 19-48% (poor), specificity 92-100% (excellent) for detecting UTI. 1
• Positive nitrite strongly suggests gram-negative bacteria (E. coli, Proteus, Klebsiella) that convert dietary nitrates to nitrites. 1
• Negative nitrite does NOT rule out UTI, particularly in infants who void frequently (shorter bladder dwell time). 1
• Requires adequate dietary nitrate intake and sufficient bladder dwell time (≥4 hours) for bacterial conversion. 1

Combined Testing

• Leukocyte esterase OR nitrite positive achieves 93% sensitivity and 72% specificity for UTI. 1, 3
• Both leukocyte esterase AND nitrite negative effectively rules out UTI with 90.5% negative predictive value. 1

Microscopic Examination

White Blood Cells (Pyuria)

• Diagnostic threshold: ≥10 WBCs/high-power field in spun urine sediment. 1
• Sensitivity 73-90%, specificity 47-81% depending on threshold used. 3
• Higher WBC counts increase specificity: >50 WBCs/HPF = 71% specificity; >100 WBCs/HPF = 86% specificity. 1
• Absence of pyuria has 83-95% negative predictive value for excluding UTI. 1

Red Blood Cells (Hematuria)

• Microscopic hematuria defined as ≥3 RBCs/high-power field on properly collected specimen. 4
• Should be confirmed in two of three properly collected specimens before initiating evaluation. 4
• Dipstick testing for blood has limited specificity (65-99%) due to false-positives from hemoglobinuria, myoglobinuria, or medications. 4
• Gross hematuria requires urgent evaluation due to 7.2 odds ratio for urologic cancer. 4

Bacteria

• Presence of bacteria in fresh, Gram-stained uncentrifuged urine correlates with ≥10⁵ CFU/mL. 1
• Microscopy for bacteria has 81% sensitivity and 83% specificity. 3
• Mixed bacterial flora indicates contamination, not true infection. 1

Epithelial Cells

• High epithelial cell counts indicate contamination, causing false-positive leukocyte esterase results. 1
• If repeat specimen still shows high epithelial cells, consider catheterization for definitive specimen. 1

Clinical Decision Algorithm

Step 1: Assess for Specific Urinary Symptoms

• Required symptoms for UTI diagnosis: dysuria, urinary frequency, urgency, fever >38.3°C, or gross hematuria. 1
• If no specific urinary symptoms present: Do NOT order urinalysis or culture; asymptomatic bacteriuria should not be treated. 1
• Non-specific symptoms alone (confusion, falls, functional decline in elderly) do NOT justify UTI testing without urinary symptoms. 1

Step 2: Ensure Proper Specimen Collection

• Midstream clean-catch in cooperative adults. 1
• In-and-out catheterization for women unable to provide clean specimens. 1
• Catheterization or suprapubic aspiration in infants/young children (2-24 months). 5, 1
• Process within 1 hour at room temperature or 4 hours if refrigerated to maintain accuracy. 1

Step 3: Interpret Urinalysis Results

If Both Leukocyte Esterase AND Nitrite Negative:

• UTI effectively ruled out in most populations (<0.3% probability). 5, 1
• No further testing needed unless strong clinical suspicion persists. 1

If Either Leukocyte Esterase OR Nitrite Positive:

• Combined with typical symptoms in healthy nonpregnant adults: Treat as uncomplicated cystitis without culture. 1
• In febrile infants <2 years, pregnant women, or suspected pyelonephritis: Obtain urine culture before antibiotics. 5, 1

Step 4: Urine Culture Interpretation

• Definitive UTI diagnosis requires BOTH: 1
  • Urinalysis suggesting infection (pyuria and/or bacteriuria)
  • Positive culture with ≥50,000 CFU/mL of single uropathogen (pediatric threshold) 5, 1
• Pure growth of single organism indicates true infection. 1
• Mixed flora indicates contamination; repeat with proper collection technique. 1

Special Population Considerations

Pediatric Patients (2-24 months)

• 10-50% of culture-proven UTIs have false-negative urinalysis, so culture is mandatory in febrile infants regardless of urinalysis results. 1
• Require both abnormal urinalysis AND ≥50,000 CFU/mL on culture for diagnosis. 5, 1
• Bag-collected specimens have only 15% positive predictive value; require catheterization or suprapubic aspiration for confirmation. 1

Elderly/Long-Term Care Residents

• 15-50% prevalence of asymptomatic bacteriuria with pyuria; presence of pyuria has low predictive value. 1
• Evaluate only with acute onset of specific urinary symptoms; do NOT screen or treat asymptomatic bacteriuria. 1
• Absence of pyuria effectively excludes bacteriuria in this population. 1

Catheterized Patients

• Asymptomatic bacteriuria and pyuria nearly universal; do NOT screen or treat. 1
• Reserve testing for symptomatic patients with fever, hypotension, or specific urinary symptoms. 1

Critical Pitfalls to Avoid

• Never diagnose UTI based on urinalysis alone without clinical symptoms; pyuria alone has exceedingly low positive predictive value. 1
• Never treat asymptomatic bacteriuria (except in pregnant women or pre-urologic procedures); provides no benefit and increases resistance. 1
• Never rely solely on dipstick without microscopic confirmation for hematuria evaluation. 4
• Never attribute hematuria solely to anticoagulation without further investigation. 4
• Never delay culture collection when indicated; always obtain before starting antibiotics. 1
• Never ignore medication history: Phenazopyridine (Azo dyes) causes color interference and requires 48-72 hour discontinuation before urinalysis. 4
• Never assume cloudy/smelly urine equals infection in elderly patients without specific urinary symptoms. 1